Methods for encoding decoding of a data flow representing of an omnidirectional video

ABSTRACT

A method of encoding and a device for encoding a data stream representative of an omnidirectional video and, correlatively, a method for decoding and a device for decoding a data stream representative of an omnidirectional video. The data stream is representative of an omnidirectional video and includes encoded data of at least one base layer representative of a 2D or 3D video representative of a view of a scene captured by the omnidirectional video, and encoded data of at least one enhancement layer representative of the omnidirectional video. The least one enhancement layer is encoded by prediction relative to the at least one base layer.

1. FIELD OF THE INVENTION

The invention is situated in the field of video compression, and moreparticularly that of techniques for encoding and decoding immersive oromnidirectional (e.g. 180°, 360° in 2D or 3D) video.

2. PRIOR ART

An omnidirectional video can be used to represent a scene from a centralpoint and to do so in every direction. The term “360° video content” isused when the totality of the field is used. A subset of the field canalso be captured, for example covering only 180°. The content can becaptured monoscopically (2D) or stereoscopically (3D). This type ofcontent can be generated by assembling sequences of images captured bydifferent cameras or else it can be generated synthetically by computer(e.g. in VR video games). The images of such a video content enable therendering, via an appropriate device, of the video along any directionwhatsoever. A user can control the direction in which the captured sceneis displayed and navigates continuously in every possible direction.

Such 360° video contents can for example be rendered by using a virtualreality helmet offering the user an impression of immersion in the scenecaptured by the 360° video content.

Such 360° video contents necessitate reception devices adapted to thistype of content (a virtual reality helmet for example) in order to offerthe functions of immersion and control of the displayed view by theuser.

However most currently used video content receivers are not compatiblewith this type of 360° video content and enable the rendering of onlyclassic 2D or 3D video contents. Indeed, the rendering of a 360° videocontent necessitates the application of geometrical transforms to theimages of the video in order to render the desired viewing direction.

Thus the broadcasting of 360° video contents is not backwards-compatiblewith the existing fleet of video receivers and is limited solely toreceivers adapted to contents of this type.

However it is observed that the content captured specifically for a 360°video broadcast can have been already captured for a 2D or 3D videobroadcast. In this case, it is the totality of the 360° contentprojected on a plane that is broadcast.

In addition, the simultaneous broadcasting of a same content captured indifferent formats (2D or 3D and 360°) to address the different videoreceivers is costly in terms of bandwidth, since it is necessary to sendas many video streams as there are possible formats, namely 2D, 3D, 360°views of the same captured scene.

There is therefore a need to optimize the encoding and the broadcastingof omnidirectional video contents, representative of a part (180°) of ascene or the totality (360°) of a scene and to do so monoscopically (2D)or stereoscopically (3D).

There are techniques of video encoding by layers, known as scalablevideo encoding, used to encode a 2D video stream in several successivelayers of refinements offering different levels of rebuilding of the 2Dvideo. For example, spatial scalability enables the encoding of a videosignal in several layers of increasing spatial resolution. Scalabilityin terms of PSNR (Peak Signal to Noise Ratio) enables the encoding of avideo signal for a fixed spatial resolution in several layers of risingquality. Scalability in the colorimetric space enables the encoding of avideo signal in several layers represented in increasingly widercolorimetric spaces.

However, none of the existing techniques enables the generation of avideo data stream representative of a scene that can be decoded by aclassic 2D or 3D video decoder as well as by a 360° video decoder.

The US document 2016/156917 describes a method for the scalable encodingof a video that can be a multiview video and wherein each view of themultiview video is encoded in a layer of the stream and predicted byanother view of the multiview video.

3. SUMMARY OF THE INVENTION

The invention improves on the prior art. To this effect, it concerns amethod for encoding a data stream representative of an omnidirectionalvideo, comprising:

-   -   the encoding in said stream of at least one base layer        representative of a 2D or 3D video, the 2D or 3D video being        representative of a view of a same scene captured by the        omnidirectional video (360°, 180° etc),    -   the encoding in said stream of at least one enhancement layer        representative of the omnidirectional video, the at least one        enhancement layer being encoded by prediction relative to the at        least one base layer.

The invention thus reduces the cost of transmission of the video streamswhen the video contents must be transmitted in 2D view as well as in360° view or in 3D view and in 3D-360° view. Thus, a classic 2D or 3Dvideo decoder will decode only the base layer or one of the base layersto rebuild a 2D or 3D video of the scene and a compatible 360° decoderwill decode the base layer or layers and at least one enhancement layerto rebuild the 360° video. The use of a prediction of the at least onebase layer to encode the enhancement layer thus reduces the cost ofencoding the enhancement layer.

Correlatively, the invention also concerns a method for decoding a datastream representative of an omnidirectional video, comprising:

-   -   the decoding, from said stream, of at least one base layer        representative of a 2D or 3D video, the 2D or 3D video being        representative of a view of a same scene captured by the        omnidirectional video,    -   the decoding, from said stream, of at least one enhancement        layer representative of the omnidirectional video, the at least        one enhancement layer being decoded by prediction relative to        the at least one base layer.

The term “omnidirectional video” herein is understood to mean equallywell a video of a scene, for which the totality of the field (360°) iscaptured and a video of a scene for which a sub-part of the 360° fieldis captured, for example 180°, 160°, 255.6°, or the like. Theomnidirectional video is therefore representative of a scene captured onat least one continuous part of the 360° field.

According to one particular embodiment of the invention, the predictionof the enhancement layer relative to the at least one base layercomprises, in order to encode or rebuild at least one image of theenhancement layer:

-   -   the generating of a reference image obtained by geometrical        projection on the reference image of an image, called a base        image, rebuilt from the at least one base layer,    -   the storing of said reference image in a memory of reference        images of the enhancement layer.

Advantageously, the prediction in the enhancement layer is carried outby the addition, during the encoding or decoding of an image of theenhancement layer, of a reference image in which the images rebuilt frombase layers are projected. Thus, a new reference image is added into thememory of reference images of the enhancement layer. This new referenceimage is generated by geometrical projection of all the base imagesrebuilt from the base layers at a time instant.

According to another particular embodiment of the invention, the datastream comprises a piece of information representative of a type ofgeometrical projection used to represent the omnidirectional video.

According to another particular embodiment of the invention, the viewpresented by the 2D or 3D video is a view extracted from theomnidirectional video.

According to another particular embodiment of the invention, the datastream comprises a piece of information representative of a type ofgeometrical projection used to extract a view of the omnidirectionalvideo and of its parameters of location.

According to one variant, such a piece of information representative ofparameters of projection and of location of said base image is encodedin the data stream in each image of the 360° video. Advantageously, thisvariant is used to take account of a shift in the scene of a viewserving as a prediction for the enhancement layer. For example, theimages of the video of the base layer can correspond to images capturedwhile moving in the scene, for example to track an object in motion inthe scene. For example, the view can be captured by a camera in motionor successively by several cameras located at different viewpoints inthe scene, to track a ball or a player during a football match forexample.

According to another particular embodiment of the invention, the datastream comprises at least two base layers, each base layer beingrepresentative of a 2D or 3D video, each base layer being respectivelyrepresented by a view of the scene, the at least two base layers beingencoded independently of each other.

Thus, it is possible to have several independent base layers in thestream enabling several 2D or 3D views of the 360° video to be rebuiltindependently.

According to another particular embodiment of the invention, an image ofthe enhancement layer is encoded by means of a group of tiles, each tilecovering a region of the image of the enhancement layer, each regionbeing distinct and separated from the other regions of the image of theenhancement layer, each tile being encoded by prediction relative to theat least one base layer. The decoding of the enhancement layer comprisesthe rebuilding of a part of the image of the enhancement layer, therebuilding of said part of the image comprising the decoding of thetiles of the enhancement layer covering the part of the image of theenhancement layer to be rebuilt, and the decoding of the at least onebase layer comprising the decoding of the base layers used to predictthe tiles covering the part of the image of the enhancement layer to berebuilt.

Such a particular embodiment of the invention enables the rebuilding ofonly one part of the omnidirectional image and not the entire image.Typically, only the part being viewed by the user is rebuilt. Thus, itis not necessary to decode all the base layers of the video stream oreven send them to the receiver. Indeed, with a user being unable tosimultaneously see the entire image of the omnidirectional video, it ispossible to encode an omnidirectional image by a tile mechanism enablingthe independent encoding of the regions of the omnidirectional image soas to then make it possible to decode only those regions of theomnidirectional image that are visible to the user.

Through the particular embodiment of the invention, the independentencoding of the base layers thus makes it possible to rebuild the tilesof the omnidirectional image separately and to limit the complexity whendecoding by avoiding the decoding of unnecessary base layers.

Advantageously, for each tile of the enhancement layer to be decoded, apiece of information identifying the at least one base layer used topredict the tile is decoded from the data stream.

The invention also relates to a device for encoding a data streamrepresentative of an omnidirectional video. The encoding devicecomprises means of encoding in said stream of at least one base layerrepresentative of a 2D or 3D video, the 2D or 3D video beingrepresentative of a view of a same scene captured by the omnidirectionalvideo, and means of encoding, in said stream, at least one enhancementlayer representative of the omnidirectional video, said means ofencoding the enhancement layer comprising means of prediction of theenhancement layer relative to the at least one base layer.

The invention also relates to a device for decoding a data streamrepresentative of an omnidirectional video. The decoding devicecomprises means for the decoding, in said stream, of at least one baselayer representative of a 2D or 3D video, the 2D or 3D video beingrepresentative of a view of a same scene captured by the omnidirectionalvideo, and means of decoding, in said stream, at least one enhancementlayer representative of the omnidirectional video, said means fordecoding the enhancement layer comprising means of prediction of theenhancement layer representative to the at least one base layer.

The encoding device and decoding device respectively are especiallyadapted to implementing the method of encoding and decoding respectivelydescribed here above. The encoding device and decoding devicerespectively could of course comprise the different characteristics ofthe encoding method and decoding method respectively, according to theinvention. Thus, the characteristics and advantages of this encoding anddecoding device respectively are the same as those of the encoding anddecoding method respectively and are not described in more ample detail.

According to one particular embodiment of the invention, the decodingdevice is comprised in a terminal.

The invention also relates to a signal representative of anomnidirectional video comprising encoded data of at least one base layerrepresentative of a 2D or 3D video, the 2D or 3D video beingrepresentative of a view of a same scene captured by the omnidirectionalvideo and encoded data of at least one enhancement layer representativeof the omnidirectional video, the at least one enhancement layer beingencoded by prediction relative to the at least one base layer.

According to one particular embodiment of the invention, an image of theenhancement layer is encoded by means of a group of tiles, each tilecovering a region of the image of the enhancement layer, each regionbeing distinct and separated from the other regions of the image of theenhancement layer, each tile being encoded by prediction relative to theat least one base layer. According to one particular embodiment of theinvention, the signal also comprises for each tile a piece ofinformation identifying the at least one base layer used to predict thetile. Thus, only the base layers needed for decoding a tile to bedecoded are decoded, thus optimizing the use of the resources of thedecoder.

The invention also relates to a computer program comprising instructionsto implement the method of encoding or the method of decoding accordingto any one of the particular embodiments described here above when saidprogram is executed by a processor. Such a program can use anyprogramming language whatsoever. It can be downloaded from acommunications network and/or recorded on a medium readable by computer.This program can use any programming language whatsoever and be in theform of source code, object code or intermediate code between sourcecode and object code, such as in a partially compiled form or in anyother desirable form whatsoever.

According to yet another aspect, the invention concerns a recordingsupport or medium or information support or medium readable by acomputer, comprising instructions of a computer program such as ismentioned here above. The recording media mentioned here above can beany entity or device capable of storing the program. For example, themedium can comprise a storage means such as a read-only memory (ROM)type memory, for example a CD-ROM or a microelectronic circuit ROM, aflash memory mounted on a detachable storage medium, such as a USBstick, or again a magnetic mass memory of the hard-disk drive (HDD) orsolid-state drive (SSD) type or a combination of memories workingaccording to one or more data-recording technologies. Furthermore, therecording medium can correspond to a transmissible medium such as anelectrical or optical signal that can be conveyed via an electrical oroptical cable, by radio or by other means. In particular, the proposedcomputer program can be downloaded from an Internet type network.

As an alternative, the recording medium can correspond to an integratedcircuit into which the program is incorporated, the circuit beingadapted to the execution of a use in the execution of the method inquestion.

The encoding or decoding method according to the invention can thereforebe implemented in various ways, especially in wired form or in softwareform.

4. LIST OF FIGURES

Other features and advantages of the invention shall appear more clearlyfrom the following description of one particular embodiment, given byway of a simple illustratory and non-exhaustive example and from theappended drawings, of which:

FIG. 1A illustrates the steps of the method of encoding according to oneparticular embodiment of the invention,

FIG. 1B illustrates an example of a signal generated according to themethod of encoding implemented according to one particular embodiment ofthe invention,

FIG. 2A illustrates an image of a view of a scene captured by a 360°video encoded in a base layer,

FIG. 2B illustrates the image illustrated in FIG. 2A projected in thereferential of an image of the 360° video,

FIG. 2C illustrates an image of the 360° video encoded in an enhancementlayer,

FIGS. 2D and 2E each illustrate an image of two views of a scenecaptured by a 360° video and each encoded in a base layer,

FIG. 2F illustrates the images of two views illustrated in FIGS. 2D and2E projected in the referential of an image of the 360° video,

FIG. 2G illustrates an image of the 360° video encoded in an enhancementlayer,

FIG. 3 illustrates steps of the method of decoding according to oneparticular embodiment of the invention,

FIG. 4A illustrates an example of an encoder configured to implement themethod of encoding according to one particular embodiment of theinvention,

FIG. 4B illustrates a device adapted to implementing the method ofencoding according to another particular embodiment of the invention,

FIG. 5A illustrates an example of a decoder configured to implement themethod of decoding according to one particular embodiment of theinvention,

FIG. 5B illustrates a device adapted to implementing the method ofdecoding according to another particular embodiment of the invention,

FIGS. 6A and 6B respectively illustrate an image of the 360°omnidirectional video encoded by independent tiles and a reference imagegenerated from two views of two base layers and used to encode the imageof FIG. 6A,

FIGS. 7A-C respectively illustrate a projection in a 2D plane of a 360°omnidirectional video with cubemap type projection, a 3D sphericalrepresentation in an XYZ referential of the 360° omnidirectional videoand a view extracted from the 360° immersive content in a 2D planeaccording to a rectilinear projection,

FIG. 7D illustrates the relationship between different geometricalprojections,

FIG. 8 illustrates the procedure for building the reference image.

The images of FIGS. 2A, C-E and G and of FIGS. 7A-B are extracted from360° videos made available by LetInVR within the framework of the JVET(Joint Video Exploration Team), JVT-D0179: Test Sequences for VirtualReality Video Coding from Letin VR, 15-21 Oct. 2016).

5. DESCRIPTION OF ONE EMBODIMENT OF THE INVENTION

5.1 General Principle

The general principle of the invention is that of encoding a data streamscalably, thus making it possible to rebuild and render a 360° videowhen a receiver is adapted to receiving and rendering such a 360° videoand rebuilding and rendering a 2D or 3D video when the receiver isadapted only to rendering a 2D or 3D video.

In order to reduce the cost of transmission of a stream comprising the2D or 3D video as well as the 360° video, according to the invention,the 2D or 3D video is encoded in a base layer and the 360° video isencoded in an enhancement or improvement layer predicted from the baselayer.

According to one particular embodiment of the invention, the stream cancomprise several base layers each corresponding to a 2D or 3D videocorresponding to a view of the scene. The enhancement layer is thusencoded by prediction on the basis of all or a part of the base layerscomprised in the stream.

5.2 Examples of Implementation

FIG. 1A illustrates steps of the method of encoding according to oneparticular embodiment of the invention. According to this particularembodiment of the invention, a 360° video is encoded scalably byextracting views from a 360° video and by encoding each view in a baselayer. The term “view” is understood here to mean a sequence of imagesacquired from a viewpoint of the scene captured by the 360° video. Sucha sequence of images can be a sequence of monoscopic images in the caseof a 360° video in 2D or a sequence of stereoscopic images in the caseof a 360° video in 3D. In the case of a sequence of stereoscopic images,each image comprises a left-hand view and a right-hand view encodedjointly for example in the form of an image generated by means ofleft-hand and right-hand views placed side by side or one above theother. The encoder encoding such a sequence of stereoscopic images in abase layer or an enhancement layer will then encode each imagecomprising a left-hand view and a right-hand view as a classic sequenceof 2D images.

Here below, we describe an embodiment in which the omnidirectional videois a 360° video in 2D.

Here we describe an embodiment where two base layers are used to encodethe enhancement layer. Generally, the method described here applies tothe case where a number of views N, with N greater than or equal to 1,is used for the encoding of the enhancement layer.

The number of base layers is independent of the number of views used togenerate the 360° video. The number of base layers encoded in thescalable data stream is for example determined during the production ofthe content or it can be determined by the encoder for purposes ofoptimizing the bit rate.

During the steps 10 and 11, a first and a second view are extracted fromthe 360° video. The views [1] and [2] are respectively encoded during anencoding step 12 for encoding a base layer BL[1] an encoding step 13 forencoding a base layer BL[2].

In one particular embodiment described here, the base layers BL[1] andBL[2] are encoded independently of one another, i.e. there is nodependence of encoding (prediction, encoding context, etc.) between theencoding of the images of the base layer BL[1] and the encoding of theimages of the base layer BL[2]. Each base layer BL[1] or BL[2] isdecodable independently of the others.

According to another particular embodiment, it is possible to encode thebase layers BL[1] and BL[2] dependently, for example to gain incompression efficiency. However, this particular embodiment of theinvention requires that the decoder should be capable of decoding bothbase layers to render a classic 2D video.

Each encoded/rebuilt image of the base layers BL[1] and BL[2] is thenprojected (steps 14 and 15 respectively) geometrically onto a samereference image I_(ref). The result of this is a partially filledreference image that contains the samples interpolated from theprojected view or views of the base layer. The building of the referenceimage is described in greater detail with reference to FIG. 8.

FIGS. 2A-2C illustrate one embodiment in which a single base layer isused. According to this embodiment, the images of the 360° video have aspatial resolution of 3840×1920 pixels and are generated by anequirectangular projection and the 360° image sequence has a frequencyof 30 images per second. FIG. 2C illustrates an image of the 360° videoat a time instant t encoded in the enhancement layer.

An image at the time instant t of the view extracted from the 360° videois illustrated in FIG. 2A. Such a view is for example extracted from the360° video by means of the following coordinates: yaw=20°, pitch=5°,horizontal FOV (field of view)=110° and vertical FOV=80°, the spatialresolution of the images of the extracted view is 1920×960 pixels andthe time frequency is 30 images per second. The yaw and pitchcoordinates correspond to the coordinates of the center (P in FIG. 2B)of the geometrical projection of an image of the view of the base layer,the yaw and pitch coordinates correspond respectively to the angle θ andthe angle φ of the point P in the pivot format illustrated in FIG. 7B.The horizontal FOV and vertical FOV parameters correspond respectivelyto the horizontal and vertical sizes of an image of the extracted viewcentered on the point P in the pivot format illustrated in FIG. 7B; thisimage of the extracted view is represented in FIG. 7C.

FIG. 2B illustrates the reference image I_(ref) used to predict theimage of the 360° video at the instant t after equirectangulargeometrical projection of the image of the base layer illustrated inFIG. 2A.

FIGS. 2D-2G illustrate an embodiment in which two base layers are used.According to this embodiment, the images of the 360° video have aspatial resolution of 3840×1920 pixels and are generated by anequirectangular projection and the 360° image sequence has a frequencyof 30 images per second. FIG. 2G illustrates an image of the 360° videoat a time instant t encoded in the enhancement layer.

An image at the time instant t of a first view extracted from the 360°video is illustrated in FIG. 2D. This first view is for exampleextracted from the 360° video by means of the following coordinates:yaw=20°, pitch=5°, horizontal FOV (field of view)=110° and verticalFOV=80°; the spatial resolution of the images of the first extractedview is 1920×960 pixels and the time frequency is 30 images per second.

An image at the time instant t of a second view extracted from the 360°video is illustrated in FIG. 2E. This second view is for exampleextracted from the 360° video using the coordinates: yaw=20°, pitch=5°,horizontal FOV (field of view)=110° and vertical FOV=80°; the spatialresolution of the images of the first extracted view is 1920×960 pixelsand the time frequency is 30 images per second.

FIG. 2F illustrates the reference image I_(ref) used to predict theimage of the 360° video at the instant t after equirectangulargeometrical projection of the images of the first view and of the secondview illustrated respectively in FIGS. 2D and 2E.

In order to project the rebuilt images of the base layers in thereference image, the following steps of geometrical transformation areapplied.

The representation of a 360° omnidirectional video in a plane is definedby a geometrical transformation characterizing the way in which a 360°omnidirectional content represented in a sphere is adapted to arepresentation in a plane. The spherical representation of the data isused as a pivot format; it makes it possible to represent the pointscaptured by the omnidirectional video device. Such an XYZ 3D sphericalrepresentation is illustrated in FIG. 7B.

For example, the 360° video is represented by means of anequirectangular geometrical transformation that can be seen as aprojection of the points on a cylinder surrounding the sphere. Othergeometrical transformations are of course possible, for example acubemap projection corresponding to a projection of points on a cubeenclosing a sphere. The faces of the cubes being finally unfolded on toa plane to form the 2D image. Such a cubemap projection is for exampleillustrated in FIG. 7A.

FIG. 7D illustrates a more detailed view of the relationship between thedifferent formats mentioned here above. The passage from anequirectangular format A to a cubemap format B is done through a pivotformat C characterized by a representation of the samples in a sphericalXYZ system illustrated in FIG. 7B. In the same way, the extraction of aview D from the format A is done through this pivot format C. Theextraction of a view of the immersive content is characterized by ageometrical transformation, for example by making a rectilinearprojection of the points of the sphere along a plane illustrated by theplane ABCD in FIG. 7C. This projection is characterized by parameters oflocation such as yaw, pitch and the horizontal and vertical field ofview (FOV). The mathematical properties of these different geometricaltransformations are documented in the document JVET-G1003 (“Algorithmdescriptions of projection format conversion and video quality metricsin 360Lib Version 4”, Y. Ye, E. Alshina, J. Boyce, JVET of ITU-T SG16WP3 and ISO/IEC JTC 1/SC 29/WG 11, 7th meeting, Turin, IT, 13-21 Jul.2017).

FIG. 8 illustrates the different steps enabling the passage between twoformats. A table of correspondence is first of all built at E80 in orderto place the position of each sample in the destination image (I_(ref)),in correspondence with its corresponding position in the source format(corresponding to the rebuilt images of the base layers BL[1] and BL[2]in the example described in FIG. 1A). For each position (u,v) in thedestination image, the following steps apply:

-   -   At E81: passage of the coordinates (u,v) of the destination        image into the pivot system XYZ.    -   At E82: projection of the XYZ coordinates of the pivot system in        the source image (u′,v′).    -   At E83: updating the table of correspondence relating the        positions in the destination format and in the source format.

Once the table of correspondence is built, the value of each pixel (u,v)in the destination image (I_(ref)) is interpolated relative to the valueof the corresponding positive (u′,v′) in the source image during a stepE84 (corresponding to the rebuilt images of the base layers BL[1] andBL[2] in the example described with reference to FIG. 1A). Aninterpolation can be done (u′v′) before assigning the value in applyinga Lanczos type interpolation filter on the decoded image of the baselayer at the position placed in correspondence.

At a step 16 of the encoding method illustrated in FIG. 1A, the 360°video is encoded in an enhancement layer EL by prediction relative tothe base layers BL[1] and BL[2] in using the reference image I_(ref)generated from the base layers.

At a step 17, the data encoded during the steps 12, 13 and 16 aremultiplexed in order to form a binary stream comprising the encoded dataof the base layers BL[1] and BL[2] and the enhancement layer EL. Theprojection data used to build the reference image I_(ref) are alsoencoded in the binary stream and transmitted to the decoder.

The encoding steps 12, 13 and 16 can advantageously be implemented bystandard video encoders, for example by a standard scalable SHVC encoderof the HEVC standard.

FIG. 1B illustrates an example of a binary stream generated according tothe method described with reference to FIG. 1A. In this example, thebinary stream comprises:

-   -   the encoded data of the base layers BL[1] and BL[2],    -   a piece of information PRJ representative of the type of        geometrical projection used to represent the omnidirectional        content, for example a value indicating an equirectangular        projection,    -   a piece of information PRJ_B1, PRJ_B2 respectively,        representative of the projection used to extract the view and        its location parameters in the 360° video from the view of the        base layer BL[1] and BL[2] respectively.

The information representative of the projection and location parametersof a view of the base layer can for example be encoded in the form ofcoordinates of the view (yaw, pitch, HFOV, VFOV) matched with the typeof projection (rectilinear projection) used to extract the view.

The information representative of the parameters of projection andlocation of a view of a base layer can be encoded only once in thebinary stream. It is thus valid for the entire image sequence.

The information representative of the parameters of projection andlocation of a view of a base layer can be encoded several times in thebinary stream, for example at each image, or at each group of images. Itis thus valid only for one image or one group of images.

When the information representative of the parameters of projection andlocation of a view is encoded at each image, such a variant procures theadvantage wherein the view extracted at each instant in time of thesequence can correspond to a view of an object that is in motion in thisscene and is tracked in the course of time.

When the information representative of parameters of projection andlocation of a view is encoded for a group of images, such a variantprocures the advantage wherein the video sequence encoded in a baselayer can change its viewpoint in the course of time thus making itpossible to track an event via different viewpoints in the course oftime.

FIG. 3 illustrates steps of the method of decoding according to oneparticular embodiment of the invention.

According to this particular embodiment of the invention, the scalablebinary stream representative of the 360° video is demultiplexed during astep 30. The encoded data of the base layers BL[1] and BL[2] in theexample described herein are sent to a decoder to be decoded (steps 31,33 respectively).

Then, the rebuilt images of the base layers are projected (steps 32, 34respectively) similarly to the encoding method on a reference imageI_(ref) to serve as a prediction for the enhancement layer EL. Thegeometrical projection is carried out from projection data provided inthe binary stream (type of projection, information on projection and onlocation of the view).

The encoded data of the enhancement layer EL are decoded (step 35) andthe images of the 360° video are rebuilt in using the reference imagesI_(ref) generated from geometrical projections made on the base layers,as specified here above.

The scalable binary stream representative of the 360° video thus makesit possible to address any type of receiver. Such a scalable stream alsomakes each receiver capable of decoding and rebuilding a 2D video or a360° video according to its capacities.

According to the decoding method described here above, classic receiverssuch as PCs, television sets, tablets, etc. will decode only one baselayer and render a sequence of 2D images, while receivers adapted to360° video such as virtual reality helmets, smartphones, etc. willdecode the base layers and the enhancement layers and render 360° video.

FIG. 4A provides a more detailed illustration of the steps encoding abase layer and an enhancement layer of the method described here aboveaccording to one particular embodiment of the invention. Here, wedescribe the case of the encoding of an enhancement layer encoding a360° omnidirectional video by prediction from a base layer encoding aview k.

Each image of the view k to be encoded is sub-divided into blocks ofpixels and each block of pixels is then encoded classically by spatialor temporal prediction in using a previously built reference image ofthe sequence of images of the view k.

Classically, a prediction module P determines a prediction for a currentblock B^(k) _(c). The current block B^(k) _(c) is encoded by spatialprediction relative to other blocks of the same image or else bytemporal prediction relative to a block of a reference image of the viewk previously encoded and rebuilt and stored in the memory MEM^(b).

The prediction residue is obtained in computing the difference betweenthe current block B^(k) _(c) and the prediction determined by theprediction module P.

This prediction residue is then transformed by a transformation module Timplementing for example a DCT (discrete cosine transform). Thetransformed coefficients of the residue block are then quantified by aquantification module Q and then encoded by the entropic encoding moduleC to form the encoded data of the base layer BL[k].

The prediction residue is rebuilt, via an inverse quantificationperformed by the module Q⁻¹ and an inverse transform performed by themodule T⁻¹ and added to the prediction determined by the predictionmodule P to rebuild the current block.

The rebuilt current block is then stored in order to rebuild the currentimage and so that this rebuilt current image can serve as a referenceduring the encoding of following images of the view k.

When the current image of the view k is rebuilt, a projection modulePROJ carries out a geometrical projection of the rebuilt image in thereference image I_(ref) of the 360° video as illustrated in FIG. 2B andaccording to the geometrical transformation described here above.

The reference image I_(ref) obtained by projection of the rebuilt imageof the base layer is stored in the memory of the enhancement layerMEM^(e).

Just as in the case of the base layer, the 360° omnidirectional video isencoded image by image and block and block. Each block of pixels isencoded classically by spatial or temporal prediction in using areference image previously rebuilt and stored in the memory MEM^(e).

Classically, a prediction module P determines a prediction for a currentblock B^(e) _(c) of a current image of the 360° omnidirectional video.The current block B^(e) _(c) is encoded by spatial prediction relativeto other blocks of the same image or else by temporal predictionrelative to a block of a previously encoded and rebuilt reference imageof the 360° video, stored in the memory MEM^(e).

According to the invention, advantageously, the current block B^(e) _(c)can also be encoded by interlayer prediction relative to a blockco-localized in the reference image I_(ref) obtained from the baselayer. For example, such a mode of encoding is reported in the encodeddata EL of the enhancement layer by an Inter encoding mode signaling atemporal encoding of the block, a zero motion vector and a referenceindex indicating the reference image of the memory MEM^(e) usedindicating the image I_(ref). These pieces of information are encoded byan entropic encoder C. Such a particular embodiment of the inventionenables the reutilization of the existing syntax of the temporalencoding modes of the existing standards. Other types of signaling areof course possible.

The mode of prediction determined to encode a current block B^(e) _(c)is for example selected from among the possible modes of prediction andby selecting the one that minimizes a bit rate/distortion criterion.

Once a prediction mode is selected for the current block B^(e) _(c), aprediction residue is obtained by computing the difference between thecurrent block B^(e) _(c) and the prediction determined by the predictionmodule P.

This prediction module is then transformed by a transformation module Timplementing for example a DCT (discrete cosine transform) type oftransform. The transformed coefficients of the residue block are thenquantified by a quantification module Q and then encoded by the entropicencoding module C to form encoded data of the enhancement layer EL.

The prediction residue is rebuilt, via an inverse quantificationperformed by the module Q⁻¹ and an inverse transform performed by themodule T⁻¹ and added to the prediction determined by the predictionmodule P to rebuild the current block.

The rebuilt current block is then stored in order to rebuild the currentimage and each rebuilt current image can serve as a reference during theencoding of following images of the 360° omnidirectional video.

The encoding has been described here in the case of a single view kencoded in a base layer. The method can be easily transposed to the caseof several encoded views in an equivalent number of base layers. Eachimage rebuilt at an time instant t of a base layer is projected on thesame reference image I_(ref) of the 360° video to encode an image of the360° video at the instant t.

FIG. 4B presents the simplified structure of an encoding device CODadapted to implementing the encoding method according to any one of theparticular embodiments of the invention described here above.

Such an encoding device comprises a memory MEM4, a processing unit UT4,equipped for example with a processor PROC4.

According to one particular embodiment of the invention, the encodingmethod is implemented by a computer program PG4 stored in a memory MEM4and managing the processing unit UT4. The computer program PG4 comprisesinstructions to implement the steps of the encoding method as describedhere above when the program is executed by the processor PROC4.

At initialization, the code instructions of the computer program PG4 arefor example loaded into a memory (not shown) and then executed by theprocessor PROC4. The processor PROC4 of the processing unit UT4implements especially the steps of the encoding method described withreference to FIG. 1A or 4A according to the instructions of the computerprogram PG4.

According to another particular embodiment of the invention, theencoding method is implemented by functional modules (P, T, Q, Q⁻¹, T⁻¹,C, PROJ). To this end, the processing unit UT4 cooperates with thedifferent functional modules and the memory MEM4 in order to implementthe steps of the encoding method. The memory MEM4 especially includesthe memories MEM^(b), MEM^(e).

The different functional modules described here above can be in hardwareor software form. In hardware form, such a functional module can includea processor, a memory and program code instructions to implement thefunction corresponding to the module when the code instructions areexecuted by the processor. In hardware form, such a functional modulecan be implemented by any type of adapted encoding circuit such as, forexample and non-exhaustively, microprocessors, digital signal processors(DSPs), applications specific integrated circuits (ASICs), fieldprogrammable gate array (FPGA) circuits, logic unit wiring.

FIG. 5A provides a more detailed illustration of the steps for decodinga base layer and an enhancement layer of the method described here aboveaccording to one particular embodiment of the invention. Here, wedescribe the case of a decoding of an enhancement layer EL encoding a360° omnidirectional video by prediction from a base layer BL[k]encoding a view k.

The view k and the 360° omnidirectional video are decoded image by imageand block by block. Classically, the data of the base layer BL[k] aredecoded by an entropic decoding module D. Then, for a current block of acurrent image to be rebuilt, a prediction residue is rebuilt via aninverse quantification of the coefficients decoded entropically by aninverse quantification module Q⁻¹ and an inverse transform performed byan inverse transform module T⁻¹. A prediction module P determines aprediction for the current block on the basis of the signaling datadecoded by the entropic decoding module D. The prediction is added tothe rebuilt prediction residue to rebuild the current block.

The rebuilt current block is then stored in order to rebuild the currentimage and so that this rebuilt current image is stored in the memory ofreference images of the base layer MEM^(b) and so that it can serve as areference during the decoding of following images of the view k.

When the current image of the view k is rebuilt, a projection modulePROJ makes a geometrical projection of the rebuilt image in thereference image I_(ref) of the 360° omnidirectional video, asillustrated in FIG. 2B and according to the geometrical transformationdescribed here above.

The reference image I_(ref) obtained by projection of the rebuilt imageof the base layer is stored in the memory of reference images of theenhancement layer MEM^(e).

The data of the enhancement layer EL are decoded by an entropic decodingmodule D. Then, for a current block of a current image to be rebuilt, aprediction residue is rebuilt via an inverse quantification of theentropically decoded coefficients implemented by an inversequantification module Q⁻¹ and an inverse transform implemented by aninverse transformation module T⁻¹. A prediction module determines aprediction for the current block from the signaling data decoded by theentropic decoding module D.

For example, the decoded syntax data indicate that the current blockB^(e) _(c) is encoded by inter-layer prediction relative to a blockco-localized in the reference image I_(ref) obtained from the baselayer. The prediction module therefore determines that the predictioncorresponds to the block co-located with the current block B^(e) _(c) inthe reference image I_(ref).

The prediction is added to the rebuilt prediction residue to rebuild thecurrent block. The rebuilt current block is then stored in order torebuild the current image of the enhancement layer.

This rebuilt image is stored in the reference image memory of theenhancement layer MEM^(e) to serve as a reference during the decoding offollowing images of the 360° video.

FIG. 5B presents the simplified structure of a decoding device DECadapted to implementing the decoding method according to any one of theparticular embodiments of the invention described here above.

Such a decoding device comprises a memory MEM5, a processing unit UT5,equipped for example with a processor PROCR5.

According to one particular embodiment of the invention, the decodingmethod is implemented by a computer program PG5 stored in a memory MEM5and driving the processing unit UT5. The computer program PG5 comprisesinstructions to implement the steps of the decoding method as describedhere above when the program is executed by the processor PROC5.

At initialization, the code instructions of the computer program PG5 arefor example loaded into a memory (not shown) and then executed by theprocessor PROC5. The processor PROC5 of the processing unit UT5especially implements the steps of the decoding method described inrelation to FIG. 3 or 5A, according to the instructions of the computerprogram PG5.

According to another particular embodiment of the invention, thedecoding method is implemented by functional modules (P, Q⁻¹, T⁻¹, D,PROJ). To this end, the processing unit UT5 cooperates with thedifferent functional modules and the memory MEM5 in order to implementthe steps of the decoding method. The memory MEM5 can especially includethe memories MEM^(b), MEM^(e).

The different functional modules described here above can be in hardwareor software form. In hardware form, such a functional module can includea processor, a memory and program code instructions to implement thefunction corresponding to the module when the code instructions areexecuted by the processor. In hardware form, such a functional modulecan be implemented by any type of adapted encoding circuit such as, forexample and non-exhaustively, microprocessors, digital signal processors(DSPs), applications specific integrated circuits (ASICs), fieldprogrammable gate array (FPGA) circuits, logic unit wiring.

According to one particular embodiment of the invention, the blocks ofan image of the enhancement layer are encoded by groups of blocks. Sucha group of blocks is also called a tile. Each group of blocks, i.e. eachtile, is encoded independently of the other tiles.

Each tile can then be decoded independently of the other tiles. Suchtiles (TE0-TE11) are illustrated in FIG. 6A representative of an imageof the 360° omnidirectional video at a time instant in which 12 tilesare defined and entirely overlap the image.

The term “independent encoding of tiles” is understood here to mean anencoding of the blocks of a tile that do not use any spatial predictionfrom a block of another tile of the image, or temporal prediction from ablock of a tile of the reference image not co-localized with the currenttile.

Each tile is encoded by temporal prediction or inter-layer prediction onthe basis of one or more of the base layers as illustrated in FIGS. 6Aand 6B. In FIGS. 6A and 6B, the tiles TE4 and TE7 are encoded byinter-layer prediction relative to the image projected in the referenceimage I_(ref) of the view 1 and of the tiles TE3 and TE6 are encoded byinter-layer projection relative to the image projected in the referenceimage I_(ref) of the view 2.

According to this particular embodiment of the invention, it can happenthat a receiver adapted to decoding and rendering a 360° video decodesonly the tiles necessary for the current zone of the 360° image viewedby a user. Indeed, during the rendering of a 360° video, a user cannot,at an instant t, view the entire image of the video, i.e. he cannot lookin all directions at the same time and can, at an instant t, view onlythe zone of the image facing his gaze.

For example, such a viewing zone is represented by the zone ZV of FIG.6A. Thus, according to this embodiment, only the base layers that haveserved for the prediction of the zone viewed by the user are decoded atthe step 31. In the example described in FIGS. 6A and 6B, only the baselayer corresponding to the view 1 is decoded during the step 31 and onlythe tiles TE4, TE5, TE7 and TE8 are decoded during the step 35 of FIG. 3on the basis of the enhancement layer EL. During the step 35, only thepart of the image of the enhancement layer corresponding to the tilesTE4, TE5, TE7 and TE8 is rebuilt. The particular embodiment describedwith reference to FIGS. 6A and 6B is described here in the case wherethe tiles of the enhancement layer EL to be decoded depend on only onebase layer (that of the view 1). According to other variants, a tile ofthe enhancement layer EL can be encoded by prediction from several baselayers, as a function for example of the choices of bit rate/distortionoptimization made during the encoding of the blocks of the enhancementlayer, a block of a tile being possibly encoded by prediction relativeto a first base layer and another block of the same tile being possiblyencoded by another base layer distinct from the first base layer. Inthis case, all the base layers used for the prediction of the blocks ofa tile of the enhancement layer must be decoded.

To this end, the stream of encoded data comprises, for each tile of theenhancement layer, a piece of information identifying the base layersused to predict the tile.

For example, for each tile, syntax elements indicating the number ofbase layers used and an identifier of each base layer used are encodedin the data stream. Such syntax elements are decoded for each tile ofthe enhancement layer to be decoded during the step 35 for decoding theenhancement layer.

The particular embodiment described here limits the use of the resourcesof the decoder and avoids the decoding of data that is unnecessarybecause it is not viewed by the user. Such an embodiment can beimplemented by any one of the encoding devices and any one of thedecoding devices described here above.

The methods of encoding and decoding described here above have beendescribed in the case where the rebuilt images of the base layers areprojected at the steps 14 and 15 of FIG. 1A and at the steps 32, 34 ofFIG. 3 on a same reference image inserted in the memory of referenceimages of the enhancement layer.

When the number of base layers is limited, for example when it is 1 or2, such a reference image has large-sized, non-defined zones, forexample set at zero by default, which then use memory resourcesunnecessarily.

According to other variants, the rebuilt images of the base layersprojected on the enhancement layer can be stored in referencesub-images. For example, a sub-image can be used for each base layer.Each sub-image is stored in association with decoding informationenabling the encoder and/or the decoder to determine the location of thesub-image in the enhancement image. Such a variant gives the advantageof saving memory space by avoiding the need for a reference image in theenhancement layer, for which the majority of the samples are zero.

Such a variant can be implemented independently of the decoder and/or ofthe encoder.

1. A method for encoding a data stream representative of anomnidirectional video, wherein the method comprises the following actsperformed by an encoding device: encoding, in said stream, at least onebase layer representative of a 2D or 3D video, the 2D or 3D video beingrepresentative of a view of a same scene captured by the omnidirectionalvideo, encoding, in said stream, at least one enhancement layerrepresentative of the omnidirectional video, the at least oneenhancement layer being encoded by prediction relative to the at leastone base layer.
 2. A method for decoding a data stream representative ofan omnidirectional video, wherein the method comprises the followingacts performed by a decoding device: decoding, from said stream, atleast one base layer representative of a 2D or 3D video, the 2D or 3Dvideo being representative of a view of a same scene captured by theomnidirectional video, decoding, from said stream, at least oneenhancement layer representative of the omnidirectional video, the atleast one enhancement layer being decoded by prediction relative to theat least one base layer.
 3. The method according to claim 1, wherein theprediction of the enhancement layer relative to the at least one baselayer comprises, in order to encode at least one image of theenhancement layer: generating a reference image obtained by geometricalprojection on said reference image of an image, called a base image,rebuilt from the at least one base layer, storing said reference imagein a non-transitory computer-readable memory of reference images of theenhancement layer.
 4. The method according to claim 3, wherein the datastream comprises a piece of information representative of a type of ageometrical projection used to represent the omnidirectional video. 5.The method according to claim 1, wherein the view represented by the 2Dor 3D video is a view extracted from the omnidirectional video.
 6. Themethod according to claim 5, wherein the prediction of the enhancementlayer relative to the at least one base layer comprises, in order toencode at least one image of the enhancement layer: generating areference image obtained by geometrical projection on said referenceimage of an image, called a base image, rebuilt from the at least onebase layer, storing said reference image in a non-transitorycomputer-readable memory of reference images of the enhancement layer;and wherein the data stream comprises a piece of informationrepresentative of parameters of projection and of location of said baseimage in an image of the omnidirectional video, said information beingused to project the base image on the reference image.
 7. The methodaccording to claim 6, wherein said piece of information representativeof the parameters of projection of location of said base image isencoded in the data stream at each image of the omnidirectional video.8. The method according to claim 1, wherein the data stream comprises atleast two base layers, each base layer being representative of a 2D or3D video, respectively representative of a view of the scene, the atleast two base layers being encoded independently of each other.
 9. Themethod of decoding according to claim 2, wherein the data streamcomprises at least two base layers, each base layer being representativeof a 2D or 3D video, respectively representative of a view of the scene,the at least two base layers being encoded independently of each other;and wherein an image of the enhancement layer is encoded by using agroup of tiles, each tile covering a region of the image of theenhancement layer, each region being distinct and separated from theother regions of the image of the enhancement layer, each tile beingencoded by prediction relative to the at least one base layer, thedecoding of the enhancement layer comprising: rebuilding a part of theimage of the enhancement layer comprising decoding the tiles of theenhancement layer covering the part of the image of the enhancementlayer to be rebuilt, and decoding the at least one base layer comprisingdecoding the base layers used to predict the tiles covering the part ofthe image of the enhancement layer to be rebuilt.
 10. The method ofdecoding according to claim 9 further comprising, for each tile of theenhancement layer to be decoded, decoding a piece of informationidentifying the at least one base layer used to predict the tile.
 11. Adevice for encoding a data stream representative of an omnidirectionalvideo, wherein the device comprises: a processor; and a non-transitorycomputer-readable medium comprising instructions stored thereon, whichwhen executed by the processor configure the device to: encode, in saidstream, at least one base layer representative of a 2D or 3D video, the2D or 3D video being representative of a view of a same scene capturedby the omnidirectional video, and encode, in said stream, at least oneenhancement layer representative of the omnidirectional video, the atleast one enhancement layer being encoded by predicting the enhancementlayer relative to the at least one base layer.
 12. A device for decodinga data stream representative of an omnidirectional video, the devicecomprises: a processor; and a non-transitory computer-readable mediumcomprising instructions stored thereon, which when executed by theprocessor configure the device to: decode, in said stream, at least onebase layer representative of a 2D or 3D video, the 2D or 3D video beingrepresentative of a view of a same scene captured by the omnidirectionalvideo, and decode, in said stream, at least one enhancement layerrepresentative of the omnidirectional video, the at least oneenhancement layer being decoded by prediction relative to the at leastone base layer.
 13. (canceled)
 14. (canceled)
 15. A non-transitorycomputer-readable medium comprising instructions stored thereon, whichwhen executed by a processor of an encoding device or respectively adecoding device configure the encoding device or respectively thedecoding device to: encode or respectively decode a data streamrepresentative of an omnidirectional video by: encoding or respectivelydecoding, in said stream, at least one base layer representative of a 2Dor 3D video, the 2D or 3D video being representative of a view of a samescene captured by the omnidirectional video, and encoding orrespectively decoding, in said stream, at least one enhancement layerrepresentative of the omnidirectional video, the at least oneenhancement layer being encoded by prediction relative to the at leastone base layer.
 16. The method according to claim 2, wherein theprediction of the enhancement layer relative to the at least one baselayer comprises, in order to rebuild at least one image of theenhancement layer: generating a reference image obtained by geometricalprojection on said reference image of an image, called a base image,rebuilt from the at least one base layer, storing said reference imagein a non-transitory computer-readable memory of reference images of theenhancement layer.
 17. The method according to claim 16, wherein thedata stream comprises a piece of information representative of a type ofa geometrical projection used to represent the omnidirectional video.18. The method according to claim 2, wherein the view represented by the2D or 3D video is a view extracted from the omnidirectional video. 19.The method according to claim 18, wherein the prediction of theenhancement layer relative to the at least one base layer comprises, inorder to rebuild at least one image of the enhancement layer: generatinga reference image obtained by geometrical projection on said referenceimage of an image, called a base image, rebuilt from the at least onebase layer, storing said reference image in a non-transitorycomputer-readable memory of reference images of the enhancement layer;and wherein the data stream comprises a piece of informationrepresentative of parameters of projection and of location of said baseimage in an image of the omnidirectional video, said information beingused to project the base image on the reference image.
 20. The methodaccording to claim 19, wherein said piece of information representativeof the parameters of projection of location of said base image isencoded in the data stream at each image of the omnidirectional video.21. The method according to claim 2, wherein the data stream comprisesat least two base layers, each base layer being representative of a 2Dor 3D video, respectively representative of a view of the scene, the atleast two base layers being encoded independently of each other.